CCC 461 Taking up St. John’s expression, “The Word became flesh”,¹ the Church calls “Incarnation” the fact that the Son of God assumed a human nature in order to accomplish our salvation in it. In a hymn cited by St. Paul, the Church sings the mystery of the Incarnation:
Have this mind among yourselves, which is yours in Christ Jesus, who, though he was in the form of God, did not count equality with God a thing to be grasped, but emptied himself, taking the form of a servant, being born in the likeness of men. And being found in human form he humbled himself and became obedient unto death, even death on a cross.²

1 Jn 1:14.
2 Phil 2:5-8; cf. LH, Saturday, Canticle at Evening Prayer.